1. Field of the Invention
The present invention relates to methods and apparatus for measuring angular rates. More particularly, this invention pertains to a method and apparatus for measuring rotation, in accordance with the Sagnac effect, utilizing a passive optical resonator.
2. Descripton of the Prior Art
The passive resonator, in addition to the active resonator (laser gyroscope) and the Sagnac interferometer, has long been recognized as suitable for measuring rates of rotation. (G. Sagnac: C. R. Acad. Sci. Paris, 95, 708 (1913)).
S. Ezekiel and S. R. Balsamo investigated the passive ring resonator at the Massachusetts Institute of Technology approximately ten (10) years ago for suitability as rate-of-rotation sensor. U.S. Pat. No. 4,135,822 relates to work performed during this study. The initial experimental results of the study were published in 1977 (Appl. Phys. Lett. 30, 478). Continuous further development of their experimental model led to development of a rate-of-rotation sensor having inertial accuracy under laboratory conditions (Opt. Lett. 6, 569 (1981)).
Although the resonators of the experimental models were executed in mirror technology, the above-referenced United States Patent discloses the possibility of a future fiber resonator. While the state of the art at the time did not allow a successful embodiment to be fabricated in optical fiber, a resonator recently has been built with the aid of a commercial high-quality coupler. This resonator was used to carry out successful measurements (R. E. Meyer et al.: Passive Fiberoptic Ring Resonator for Rotation Sensing, Preprint MIT 1983).
Parallel research work in the United States, particularly in the E. L. Ginzton Laboratory of Stanford University led to the development of a low-loss directional fiber coupler (Electron. Lett. 16, 260 (1980)). Using couplers of this type, it was possible to produce resonators having a finesse of 60-90 (see L. F. Stokes et al.: Opt. Lett. 7, 288 (1982)). Experimental investigations relating to their suitability as rate-of-rotation sensors have been recently published (see G. L. Report No. 33620, E. L. Ginzton Laboratory, Stanford University, September 1983).
The development of integrated passive resonators has also become known (see U.S. Pat. No. 4,326,803 and A. Lawrence, "The Micro-Optic Gyro", NORTHROP Precision Products Division, August 1983).
The unsuitability of mirror technology for the resonator of a rate-of-rotation sensor arises from the fact that it is difficult to maintain the axial TEM.sub.oo mode in the resonator under unfavorable environmental conditions. In contrast, lower sensitivity to temperature gradient exists in a fiber resonator in comparison with a Sagnac interferometer because of the considerably shorter fiber length required (see D. M. Shupe: Appl. Opt. 20, 286 (1981)). It is known, however, that such a ring can carry two natural states of polarisation (see B. Lamouroux et al.: Opt. Lett. 7, 391 (1982)). Coupling of these two states can result from environmental influences to produce additional noise in the output channel.
Additionally, only single-mode He-Ne lasers have been used in the past light sources. The backscatter occurring in the fiber resonator in such an arrangement is a significant cause of the interferences that disturb the useful signal. In theory, using one or more longer-wave coherent light sources may lessen the effect of such interference since the Rayleigh backscatter is inversely proportional to the fourth power of the light wavelength. Attempts to use longer-wave-semiconductor lasers have been unsuccessful as laser spectral width is too great for a good fiber resonator. A significant reduction of the spectral width of a semiconductor laser can be achieved by using an external resonator (S. Saito and Y. Yamamoto: Electr. Lett. 17, 325 (1981); M. W. Fleming and A. Mooradian: IEEE J. Quant. Electr. QE-17, 44 (1981)). By adding one or more dispersive elements, gratings and/or mirrors to the semiconductor laser or by directly coating the semiconductor laser, a light source can be obtained such that the quality of the optical resonator is enhanced. The possibility of constructing the external resonator in fiber technology also exists (IEEE Transactions on Microwave Theory and Techniques, MTT-30, No. 10, 1700 (1982)).
The problem of undesired low-frequency interferences in the useful signal due to a signal wave being mixed with the backscattered component of the returning wave always occurs whenever the two opposing light sources occupy the same longitudinal resonator mode. A known possibility of remedying this situation is the use of additional phase modulation in the optical path feeding the resonator (Sanders et al.: Opt. Lett. 6, 569 (1981)).
The occupation of two different longitudinal modes by oppositely-directed light waves brings the interference frequencies into such a high range that they no longer appear as interference. However, this leads to an extraordinarily high temperature-dependent null drift as a change in temperature changes the optical length, and thus the mode separation, of the resonator.
The mode separation .DELTA..nu. of a ring resonator having a length of L=10 m and an effective refractive index of n=1.46 is: ##EQU1## If the fiber of the ring resonator consists of quartz, the change in optical wavelength with a change in temperature is essentially determined by the relative change of the refractive index of about 1.times.10.sup.-5 /.degree.C. This results in a temperature-dependent drift of the mode separation of ##EQU2## If the rate of rotation is determined from the frequency separation of the two light waves oppositely directed in various longitudinal modes, this temperature-dependent change in frequency separation leads to a drift in the null of the rate of rotation via the familiar Sagnac relationship ##EQU3## (P: periphery of the resonator, .lambda.: light wavelength, F: area within the periphery, .DELTA..nu.s: frequency difference between oppositely directed light waves as a result of the Sagnac effect).
In the preceding example, this relationship produces an unacceptably large null drift of 132.degree./h/.degree.C. for a circular resonator having a diameter of 18 cm and a wavelength of 0.83 .mu.m.